1) Field of the Invention
Embodiments of the invention relate to communications apparatuses, networks and systems, as well as methods and computer programs for the same.
2) Description of Related Art
A communication system can be seen as a facility that enables communication sessions between two or more entities. The communications may comprise, for example, communication of voice, electronic mail (email), text message, multimedia, other data and so on. A communication system can be provided, for example, by means of a communication network and one or more compatible communication devices. The communication network may be a local network.
A user can access a communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications with a communication network or communications directly with other users.
Cellular communication systems are those in which a communications device is able to wirelessly communicate with a network. In such cellular systems a network entity, often called a base station, Node B or evolved Node B (eNB)—collectively access nodes—provide a node for communication with the communication device in one or more cells or sectors. Each access node may provide one cell in the area surrounding it. Alternatively an access node may provide a plurality of cells by dividing the area surrounding it into a plurality of sectors.
The cells in a cellular communication system may overlap. They may be of differing sizes. One large cell may cover a geographic area which additionally contains a plurality of smaller cells.
Typically the access nodes are connected to a wider communications network. This communications network may have connections to further communications networks, including the Internet.
The bandwidth available for transmissions between a device and an access node generally comprises a plurality of component carriers; and each transmission is made on one or more selected component carriers. Component carriers represent a bundle of resources, and may be narrow (i.e. few resources) or wide (many resources).
The use of the same component carriers by neighbouring access nodes (defining neighbouring, superimposed or overlapping cells) is dependent on the interference coupling between the access nodes. Interface coupling is indicative of the degree that the communications of a first access node will interfere with the communications of a second access node. This can be derived from measurements of the signal to interference plus noise ratio (SINR) of communications with a particular access node.
When a first access node is using a particular component carrier, the interference coupling between the first access node and a neighbouring access node describes the effect that the use of the component carrier by the first access node has on the SINR at the neighbouring access node. The interference coupling may be sufficiently high (and the SINR sufficiently low) that the particular component carrier is effectively unavailable to the same neighbouring access node.
There are a finite number of component carriers, and therefore, to avoid each access node adversely affecting the SINR of its neighbouring access nodes, the component carriers are allocated between the access nodes in a given area.
It has been proposed that each access node automatically selects one and only one of the component carriers as its primary component carrier (PCC) or base carrier when the access node is powered on. The primary component carrier is used for essential functions such as setting up calls between the access node and a communication device; and it has been proposed to assure the quality of the PCC for any access node, and to make the reselection of a new component carrier as the PCC as infrequent as possible.
Each access node may also autonomously select one or more additional component carriers as secondary component carriers (SCC). The main purpose of secondary component carriers is to provide additional capacity whenever possible, and relatively frequent reselection by an access node of different carriers as its secondary component carrier(s) is envisaged. The component carriers may therefore be considered shared resources, which are shared between neighbouring access nodes.
It has been proposed that each access node maintains a so-called background interference matrix (BIM), which expresses this interference coupling between neighbouring access nodes. The BIM is built locally by each access node based on measurements from the communications devices that are served by the access node. The BIM describes the interference between neighbouring access nodes.
In proposals for the Long Term Evolution—Advanced (or LTE-A) standard for cellular communications, the selection of component carriers is performed autonomously using autonomous component carrier selection or ACCS.
ACCS may provide an automatic and fully distributed mechanism for dynamic frequency re-use on a component carrier resolution. In principle, each component carrier is eligible for use in any cell provided that certain signal to interference plus noise ratio (SINR) constraints are satisfied.
Under the proposals, heavily loaded cells are provided with more secondary component carriers than lightly loaded ones. The provision of secondary component carriers is performed on a “first come first served” basis. This is effective under low or moderate load conditions where the inherently time-varying nature of traffic in each cell will help accommodate the demand for extra component carriers.
However, under high load situations, especially within dense local area networks, the competition among neighbouring access nodes for more resources coupled to the lack of more elaborate rules governing access to component carriers, can result in some cells having a limited bandwidth.
Some cells may be left with nothing but their single primary component carriers since the minimal SINR constraints that all cells use during the SCC selection prevent the use of component carriers for which there is too great an extent of interference coupling. In other words, at high loads, the number of secondary component carriers allocated to each cell at any given moment is highly dependent on the time evolution, i.e. the order in which cells have attempted the allocation of extra component carriers, which can result in unfair allocation of resources between the different cells.
This may lead to the pre-emptive selection of secondary component carriers as there are no guarantees that a cell will still be granted access to secondary component carriers after the neighbouring cells have made their choices.
Alternatively, the SINR requirements for SCC selection could be set low from the start; nonetheless this approach may entail cells sacrificing the quality (in terms of SINR) of their SCCs for the sake of the network even when it is not necessary.